The invention relates to the control of power levels of transmitted signals in telecommunication systems, in particular cellular spread spectrum systems.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and WCDMA telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS standard. This application focuses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
WCDMA is based on direct-sequence spread-spectrum techniques. Two different codes are used for separating base stations and physical channels in the downlink (base-to-terminal) direction. Scrambling codes are pseudo-noise (pn) sequences that are mainly used for separating the base stations or cells from each other. Channelization codes are orthogonal sequences that are used for separating different physical channels (terminals or users) in each cell or under each scrambling code. Since all users share the same radio resource in CDMA systems, it is important that each physical channel does not use more power than necessary. This is achieved by a transmit power control mechanism in which the terminal estimates the signal-to-interference ratio (SIR) for its dedicated physical channel (DPCH), compares the estimated SIR against a reference value, and informs the base station to adjust the base station's transmitted DPCH power to an appropriate level. WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology.
The 3GPP is working on an evolution of WCDMA known as high speed downlink packet data access (HSDPA). This enhancement to prior systems increases capacity, reduces round-trip delay, and increases peak data rates up to 8-10 Mbit/s.
Generally speaking, transport channels are used for carrying user data via a dedicated or common physical channel. For HSDPA, the transport channel is a high-speed downlink shared channel (HS-DSCH). A corresponding physical channel is denoted by high speed physical downlink shared channel (HS-PDSCH). The HS-DSCH code resources include one or more channelization codes with a fixed spreading factor of 16. In order to leave sufficient room for other required control and data bearers, up to 15 such codes can be allocated. The available code resources are primarily shared in the time domain. For example, they may be allocated to one user at a time. Alternatively, the code resources may be shared using code multiplexing. In this case, two to four users share the code resources within a same transmission time interval (TTI).
In addition to user data, it is also necessary to transmit control signaling to notify the next user equipment (UE), such as a mobile station, personal digital assistant (PDA), or other receiver, to be scheduled. This signaling is conducted on a high-speed shared control channel (HS-SCCH), which is common to all users. The HSDPA concept also calls for an additional high-speed dedicated physical control channel (HS-DPCCH) in the uplink for carrying the Channel Quality Indicator (CQI) information in addition to the H-ARQ acknowledgements.
FIG. 1 depicts a communication system, such as a WCDMA system, that includes a base station (BS) 100 handling connections with four UEs 1, 2, 3, 4 that each uses downlink (i.e., base-to-UE or forward) and uplink (i.e., UE-to-base or reverse) channels. In the downlink, BS 100 transmits to each UE at a respective power level, and the signals transmitted by BS 100 are spread using orthogonal code words. In the uplink, UE 1-UE 4 transmit to BS 100 at respective power levels. Although not shown, BS 100 also communicates with a radio network controller (RNC), which in turn communicates with a public switched telephone network (PSTN).
The signals transmitted in the exemplary WCDMA system depicted in FIG. 1 can be formed as follows. An information data stream to be transmitted is first multiplied with a channelization code and then the result is multiplied with a scrambling code. The multiplications are usually carried out by exclusive-OR operations, and the information data stream and the scrambling code can have the same or different bit rates. Each information data stream or channel is allocated a unique channelization code, and a plurality of coded information signals simultaneously modulate a radio-frequency carrier signal. At a UE (or other receiver), the modulated carrier signal is processed to produce an estimate of the original information data stream intended for the receiver. This process is known as demodulation.
Good transmit power control methods are important for WCDMA (and other) communication systems having many transmitters that transmit simultaneously to minimize the mutual interference of such transmitters while assuring high system capacity. Depending upon the system characteristics, power control in such systems can be important for transmission in the uplink, the downlink, or both. To achieve reliable reception of a signal at each UE, the SIR of the received signal should exceed a prescribed threshold for each UE. For example, as shown in FIG. 1, consider the case in which the UEs receive, respectively, four signals on a common WCDMA communication channel. Each of the signals has a corresponding energy level associated with it, namely energy levels E1, E2, E3, and E4, respectively. Also present on the communication channel is a certain level of noise (N). For a given UE 1 to properly receive its intended signal, the ratio between E1 and the aggregate levels of E2, E3, E4, and N must be above the given UE's prescribed threshold SIR.
To improve the SIR of a received signal, the power of the transmitted signal may be increased, depending on the SIR measured at the receiver. Power, however, is an important resource in a WCDMA system. Since different channels are transmitting simultaneously at the same frequency, it is important to keep the power level on each physical channel as low as possible while still maintaining an acceptable error rate of the received blocks of user data on the transport channel, i.e., block error rate (BLER).
Downlink power control can be logically divided into an “inner loop” and an “outer loop,” where the inner loop controls the SIR by sending transmission power control (TPC) commands to the base station and the outer loop controls the quality, in terms of BLER, by providing SIR references to the inner loop.
Conventional power control techniques compute one SIR reference for each transport channel, based on the BLER and the BLER reference for that transport channel, and the maximum of these SIR references is used by the inner loop. A conventional power control system is illustrated by the block diagram of FIG. 2. Each transport channel 201, 202, etc., communicating via a physical channel of the system 200 has an associated BLER controller 211, 212, etc., that determines the current BLER and compares it to a target BLER (also referred to as a BLER reference) BLERref for the channel to produce an SIR reference, SIRref, for the associated channel. The SIRref represents a target SIR for the channel. The BLERref for a channel is established by the system according to quality requirements for the data being transported over the channel, and other parameters. For example, voice data may have a higher quality requirement than text data. These quality requirements are typically signaled on a higher layer communication. A maximum one of the SIR references is determined 220 and forwarded to the SIR controller 230. The SIR controller 230 generates the inner loop power control commands 240 based on SIRref and the current SIR.
The objective of outer loop power control is to keep the BLER on each transport channel below their BLER reference while minimizing the power demands. The BLER control 211, 212, etc., for each transport channel includes a BLER estimator and controls the respective SIRref to keep the estimated BLER close to, but below the BLER reference value, BLERref, for the channel. The current BLER is estimated based on a cyclic redundancy check (CRC) of the respective transport channel, which is typically either a “1” when an incorrect block is received or a “0” when a correct block is received. The BLER can therefore be simply determined based on the ratio of incorrect blocks (having a CRC of “1”) to the overall number of blocks received. Each controller 211, 212, etc., computes a corresponding SIRref, which is updated as each new block is received. The SIRref for a given channel increases as the BLER increases, since increasing the SIR reference value results in a demand for a higher SIR of the current signal. The SIRref having the highest value among the ones from the transport channels 201, 202, etc., is compared by the SIR controller 230 to the current SIR for the physical channel of the system 200 to produce the inner loop power control commands 240 to adjust power on the physical channel. The highest SIRref is used since it corresponds to the transport channel having the highest BLER, i.e., needing the most attention in terms of increasing the SIR.
The Association of Radio Industries and Businesses' (ARIB) “Specifications of Air-Interface for 3G Mobile System” specifies a simple method for outer loop control using an algorithm referred to as a “jump algorithm.” In the jump algorithm, the BLER is represented by a CRC. The error e is the difference between the CRC and the BLER reference, as shown in Equation (1), which is integrated to obtain the SIR reference according to Equation (2).e=CRC−BLERref   (1)SIRref(k+1)=SIRref(k)+Ke(k)   (2)where K is gain applied.
The jump algorithm is updated each time a new block is received on the transport channel. The SIRref “jumps” to a higher value each time a given TTI contains a block error. The SIR reference provided by the conventional jump algorithm results in large variations. Consequently, as can be appreciated from FIG. 2, the power will have large variations. Smaller power variations can be obtained if the CRC information is filtered to produce a BLER estimate, BLERest, before being used by the controller, in which case Equation (1) becomes Equation (3). A BLER estimate can be obtained according to Equation (4).e=BLERest−BLERref.   (3)BLERest(k+1)=α·BLERest(k)+(1α−β)*CRC(k)+β·CRC(k+1)   (4)where α and β are constants, α determining the time constant of the filter chosen based on the BLERref according to, for example, α=10−0.1 BLERref, so that the estimates of the BLER are based on roughly the same number of block errors.
The problem with the conventional power control methods today, such as the jump algorithm, is that the convergence of the SIR reference is slow when the number of received blocks over a given TTI, i.e., the block rate (BLR), on the transport channel is low. This occurs because the controller is updated less and less frequently as the BLR decreases, resulting in slower convergence. This is illustrated in the graph of FIG. 3, where a jump algorithm-based controller is used to generate an SIR reference at three different BLRs, a 100% BLR 320, a 10% BLR 310, and a 1% BLR 300. As can be appreciated from the graph, as the BLR decreases, the convergence takes longer.
Slow convergence results in poor channel quality or higher than necessary power demands. It is especially problematic when the SIR reference is far from the correct level, since either the BLER or the power level will remain too high over an extended period of time, thus reducing system capacity. A typical situation where the SIR reference is too high is at initialization, where a high SIR reference is used to guarantee reception of the first data blocks. Accordingly, there is a need for innovative power control techniques that improve convergence time and thereby reduce the power level required by a physical channel.